Baseline biomarkers were selected on the basis of a priori relevance for cognitive impairment in PD and related neurodegenerative disorders and included dopamine transporter (DAT) loss in caudate and putamen (e.g., Pasquini et al., 2019), APOE4 genotype (e.g., Tan et al., 2021), and levels of α-synuclein, amyloidß42, and tau from cerebrospinal fluid (CSF; e.g., Siderowf et al., 2010; Kang et al., 2016). Here, SLC6A3 is linked to Parkinson disease.